1. Field of the Invention
The present invention relates to an electrical characteristic measuring apparatus for measuring electrical characteristics of a semiconductor chip or the like.
2. Description of the Related Art
For example, for a semiconductor wafer on which a large number of semiconductor chips are formed, electrical characteristics of semiconductor chips are measured in an inspection step. Good and defective semiconductor wafers are distinguished on the basis of the measurement results. An electrical characteristic measuring apparatus having an arrangement shown in, e.g., FIG. 1 is conventionally used as a semiconductor wafer prober. Reference numeral 1 denotes a semiconductor wafer as an object to be measured. Hundreds of semiconductor chips are formed regularly on semiconductor wafer 1. Semiconductor wafer 1 is placed on wafer table 2. Table 2 has a two-layered structure of ceramic base 2b and aluminum base 2a. Probe card 3 is arranged above table 2 to oppose semiconductor wafer 1. Probes 4 project from probe card 3. The electrical characteristics of each semiconductor chip are measured by abutting probes 4 against pads (electrodes formed in a chip) of a predetermined semiconductor chip. More specifically, probes 4 are electrically connected to the corresponding semiconductor chip. A predetermined detective current is flowed from probes 4 connected with a tester to the semiconductor chip. The electrical characteristics of the semiconductor chip are checked based on predetermined semiconductor electrical characteristics corresponding to the detective current measured from the chip. In this case, table 2 is electrically shielded from external noise by shield plate 5 mounted on the lower surface of ceramic base 2b. More specifically, shield plate 5 is grounded through cable 6, so that shield plate 5 exhibits a shielding effect. Shield plate 5 is formed of a conductive material.
Good and defective semiconductor chips are distinguished based on the above mentioned measurement results. Finally, a good or defective semiconductor wafer is determined.
However, in the above-mentioned electrical characteristic measuring apparatus, table 2 is partially formed by aluminum base 2a. Aluminum is a conductive material. For this reason, noise from devices such as a motor incorporated in the electrical characteristic measuring apparatus causes an eddy current to be generated in aluminum base 2a due to an electromagnetic induction effect. A potential of the rear surface of semiconductor wafer 1 cannot become 0 V due to the influence of the eddy current. As a result, measurement errors of electrical characteristics of the semiconductor chips occur.
When semiconductor chips constituting an object to be measured are, e.g., power transistors, table 2 is arranged as follows.
More specifically, as shown in FIG. 2, circular voltage measuring electrode 7 made of gold plating is formed at, e.g., the central portion of the surface of aluminum base 2b. Annular voltage application electrode 8 made of, e.g., gold plating, is formed around electrode 7. Electrodes 7 and 8 are formed at a predetermined interval. With this interval, electrodes 7 and 8 are electrically insulated from each other.
In another table shown in FIG. 3, semi-circular voltage measuring electrode 9 and voltage application electrode 10 are formed on the surface of aluminum base 2b at a predetermined interval so as to be electrically insulated from each other.
In the electrical characteristic measuring apparatus with the arrangement shown in FIG. 2 or 3, a good or defective semiconductor is determined as follows.
More specifically, a semiconductor wafer on which semiconductor chips (e.g., power transistors) are formed is placed on aluminum base 2b. A predetermined voltage is applied from voltage application electrode 8 or 10 to collector electrodes formed on the rear surface of the semiconductor wafer. Then, probes 4 are brought into contact with emitter and base electrodes formed on the front surface of the semiconductor wafer. A voltage applied from voltage application electrode 8 or 10 is measured by voltage measuring electrode 7 or 9. The semiconductor chips are checked based on the measurement values. As a result, a good or defective semiconductor wafer is determined.
However, distances to electrodes 8 (or 10) and 7 (or 9) differ in accordance with the positions of semi-conductor chips formed on the semiconductor wafer. In addition, each collector electrode formed on the rear surface of the semiconductor wafer has a predetermined electrical resistance. Therefore, a voltage drop caused by a difference in distances between the semiconductor chips and electrodes 8 (or 10) and 7 (or 9) is generated during measurement of electrical characteristics. As a result, measurement results vary in accordance with the positions of the semiconductor chips on the semiconductor wafer.